Metameric variation of upper molars in hominoids and its implications for the diversification of molar morphogenesis

Abstract

Metameric variation of molar size is in part associated with the dietary adaptations of mammals and results from slight alterations of developmental processes. Humans and great apes exhibit conspicuous variation in tooth morphology both between taxa and across tooth types. However, the manner in which metameric variation in molars emerged among apes and humans via evolutionary alterations in developmental processes remains largely unknown. In this study, we compare the enamel-dentine junction of the upper molars of humans-which closely correlates with morphology of the outer enamel surface and is less affected by wear-with that of the other extant hominoids: chimpanzees, bonobos, gorillas, orangutans, and gibbons. We used the morphometric mapping method to quantify and visualize three dimensional morphological variation, and applied multivariate statistical analyses. Results revealed the following: 1) extant hominoids other than humans share a common pattern of metameric variation characterized by a largely linear change in morphospace. This indicates a relatively simple graded change in metameric molar shape; 2) intertaxon morphological differences become less distinct from the mesial to distal molars; and 3) humans diverge from the extant ape pattern in exhibiting a distinct metameric shape change trajectory in the morphospace. The graded shape change and lower intertaxon resolution from the mesial to distal molars are consistent with the concept of a 'key' tooth. The common metameric pattern observed among the extant nonhuman hominoids indicates that developmental patterns underlying metameric variation were largely conserved during ape evolution. 2 Furthermore, the human-specific metameric pattern suggests considerable developmental modifications in the human lineage.

Neutral evolution of human enamel–dentine junction morphology

“…There is also considerable metameric variation in EDJ morphology, and humans frequently express greater variation along the tooth row of a single individual than between individuals (39, 44). Metameric variation in EDJ morphology increases along the molar row, consistent with studies on other dental traits in primates (25,40,41,47,59). Given this reported variation, it has become essential to understand the extent of disparity in EDJ morphology before this well-preserved phenotype can be utilized in archaeological and paleontological investigations of taxonomy and evolution.…”

“…This is particularly notable in archaic and fossil populations, as well as contemporary human populations with a nonindustrialized, processed diet, limiting the utility of these morphologies in many studies (34)(35)(36)(37). With advances in scanning technologies alleviating the need for destructive analyses (38,39), greater attention has now been placed on the internal morphology of teeth (34)(35)(36)(40)(41)(42).…”

Análisis odontométrico en dentición permanente de mestizos caucasoides de Popayán, Colombia, 2020

“…Es por ello que el mestizaje ha sido uno de los factores que posiblemente ha tenido la mayor influencia en la tendencia hacia disminución de los diámetros dentales y por tanto en el tamaño de los dientes. Al respecto, Scott e Irish [17] habían manifestado que la interacción constante de diversos genotipos durante la evolución humana ha causado una disparidad en la morfometría de los dientes relacionada con las diferencias en la expresión génica en la odontogénesis; debido a que, de acuerdo con Morita et al [18] diámetro meso-distal) y la morfología dental (rasgos morfológicos) [5].…”

“…De esta forma, el desarrollo de todas estas características ha obedecido no solo a la manera como interactúan los diferentes niveles de organización biológica (moléculas, células, estructuras celulares, tejidos, estructuras tisulares y órganos), sino también a diferentes fenómenos emergentes que presionan a los individuos de un grupo poblacional, desafiando su capacidad de adaptación a través del tiempo (fenómeno direccional transgeneracional) y espacio (factores aleatorios como migraciones, contactos, aislamientos y mestizaje) [18].…”